Python Sigma Sums

Question

I have a list of values x=[1,-1,-1,1,1,-1,1,-1,1,-1] and I have another blank list y=[ ]

I am trying to create a function that will take a sigma sum of values in x and store them in y.

For instance, y[0] should be the sum of x[0]*x[0] + x[0]*x[1] + x[0]*x[2] + ... + x[0]*x[9] .

Similarly, y[1] should be the sum of x[1]*x[0] + x[1]*x[1] + x[1]*x[2]+ ... + x[1]*x[9].

This has to be done for y[0] through y[9].

Also, in the sums, x[i]*x[i] must be zero. So for instance in y[0], x[0]*x[0] has to be zero. Similarly, in the sum for y[1], x[1]*x[1] must be zero.

This is my code, but it always gives me some sort of error regarding indices:

x=[1,-1,-1,1,1,-1,1,-1,1,-1]
y=[]
def list_extender(parameter):
    for i in parameter:
        parameter[i]*parameter[i]==0
        variable=numpy.sum(parameter[i]*parameter[:])
        if variable>0:
            variable=1
        if variable<0:
            variable=-1
        y.append(variable)
    return y

Then i run print list_extender(x) which should print list y with the sigma sums described above, but I always get an error. What I am doing wrong? The help will be highly appreciated!

Answer 1

You're doing way too much typing and computation here. Your function could be shorter and simpler if you computed the sum of x first, then used that to compute the elements of y. It'd also run faster.

Just do this:

x_sum = sum(x)
y = [item * (x_sum - item) for item in x]
# or, if you really want to store the results into an existing list y
# y[:] = [item * (x_sum - item) for item in x]

Replace sum and the list comprehension with numpy operations if you're using numpy:

import numpy as np
x = np.array([1,-1,-1,1,1,-1,1,-1,1,-1])
y = x * (x.sum() - x)